High-Mobility Flexible Transistors with Low-Temperature Solution-Processed Tungsten Dichalcogenides

The investigation of high-mobility two-dimensional (2D) flakes beyond molybdenum disulfide (MoS2) will be necessary to create a library of high-mobility solution-processed networks that conform to substrates and remain functional over thousands of bending cycles. Here we report electrochemical exfoliation of large-aspect-ratio (>100) semiconducting flakes of tungsten diselenide (WSe2) and tungsten disulfide (WS2) as well as MoS2 as a comparison. We use Langmuir–Schaefer coating to achieve highly aligned and conformal flake networks, with minimal mesoporosity (∼2–5%), at low processing temperatures (120 °C) and without acid treatments. This allows us to fabricate electrochemical transistors in ambient air, achieving average mobilities of μMoS2 ≈ 11 cm2 V–1 s–1, μWS2 ≈ 9 cm2 V–1 s–1, and μWSe2 ≈ 2 cm2 V–1 s–1 with a current on/off ratios of Ion/Ioff ≈ 2.6 × 103, 3.4 × 103, and 4.2 × 104 for MoS2, WS2, and WSe2, respectively. Moreover, our transistors display threshold voltages near ∼0.4 V with subthreshold slopes as low as 182 mV/dec, which are essential factors in maintaining power efficiency and represent a 1 order of magnitude improvement in the state of the art. Furthermore, the performance of our WSe2 transistors is maintained on polyethylene terephthalate (PET) even after 1000 bending cycles at 1% strain.

an overlapping 2LA and E 2d peak at 354 cm -1 . 3 The peak at 521 cm -1 is attributed to the Si/SiO 2 substrate. The WSe 2 (Supplementary Figure 1a, blue) has a peak at (~ 250 cm -1 ) attributed to the A 1g and E 2g Raman modes, indicating the formation of few-layer flakes. 4,5 For the MoS 2 , WS 2 and WSe 2, the J 2 and J 3 vibrational modes attributed to the metallic 1T phase are not observed. [6][7][8][9] The absent 1T phase peaks (marked by an asterisk to show their location) would be located at 224 and 289 cm -1 for MoS 2 , 385 cm -1 for WS 2 and 218 and 236 cm -1 for WSe 2 .
To compare the flakes of electrochemical exfoliation to high-shear mixing, we make two MoS 2 inks. First, the electrochemically exfoliated ink is made as described in the main text. Then, for the shear mixed MoS 2 ink, we use MoS 2 powder (Alfa Aesar, 10 mg/ml) with 1 mg/ml PVP as a stabilisation agent in DMF and shear mix with a 4-blade rotor (Silverson Model L5M) for 8 hours (8000 rpm). The rotor had a rotor-stator gap of 300 μm and a rotor diameter of 31.1 mm.
The dispersion was centrifuged at 500 rpm (24g) for 20 minutes following shear mixing to remove bulk MoS 2 . The dispersion is then size selected by centrifuging the supernatant (top 90 %) at 1000 rpm (97g) for 1 hour. Finally, the sediment is collected and redispersed in IPA to make the shear mixed MoS 2 ink. For a fair comparison between the shear mixed MoS 2 ink and electrochemically exfoliated MoS 2, the starting MoS 2 and PVP concentration were the same.
Furthermore, the centrifugation conditions and solvent used were the same. The two inks were drop-cast on separate Si/SiO 2 chips after dilution in IPA by a factor of 1:100. The samples were then annealed at 120 °C for 15 min to remove residual solvent. A Bruker Multimode 8 microscope was used to undertake AFM and analyse the thickness and lateral size of the flakes using OLTESPA R3 cantilevers in ScanAsyst mode. In supplementary figure 1b, we observe an aspect ratio of the shear mixed flakes between 4 -38, with the majority having an aspect ratio of ~10. Supplementary figure 1c shows a typical flake found in the shear-mixed MoS 2 ink with an apparent thickness of ~ 30nm. Conversely, the aspect ratio of the electrochemically exfoliated flake could be >100. Therefore it is more likely that the electrochemically exfoliated flakes will make more conformal (flake-to-flake) junctions when aligned in a network. and scattering components of the TMD inks. 11 The scattering component of the TMD inks is ~50% of the extinction spectra intensity and would suggest that the flakes have a large L. The ratio of the extinction at the B exciton peak (Ext B ) to the local minimum at 345 nm (Ext 345 ) in the MoS 2 spectra (supplementary figure 2a, black curve) can be used to measure L. 11 The Ext B /Ext 345 > 1 indicates that the flakes are large with L > 400 nm. 11 Similarly, using the ratio of the extinction at the A exciton peak (Ext A ) to the local minimum (Ext A /Ext 295 ), the L of WS 2 can be determined. 11 We find Ext A /Ext 295 > 1 indicating L > 400 nm in agreement with our AFM statistics from the main text. 11,12 Supplementary Note 3 Hysteresis in electrochemical transistors is typically caused by low ionic mobility. 13 In our transfer curves, we note minimal hysteresis after conducting a forward (from -3 V g to 3 V g ) and backward V g sweep, probably due to the low network thickness. 14  We note that this measured capacitance includes a small parallel background capacitance associated with double layer formation on the electrodes. This is given by

Microscopy of MoS
where  (usually taken as 1 nm) is the double layer thickness and  r is usually taken as 10. 15 Here the electrode area is A electrodes =0.75 mm 2 .
The channel capacitance (C) is given by . This value was then normalised to the channel area (0.0055 cm 2 , calculated from the product of channel length, 50 m and width 11 mm) and plotted versus scan rate in Supplementary Figure 8b. This data shows the areal capacitance to fall off with increasing scan rate as is commonly seen for supercapacitors. 16 We can check this data by noting that this dependence of capacitance on scan rate is described Where  is the characteristic time associated with charge or discharge. The solid line in Supplementary Figure 8b is a fit to this data which shows very good agreement with the data.
The fit gives a value of the capacitance at infinitely low rate of C Low-rate =28 F/cm 2 . This value is a measure fo the capacitance unhindered by rate effects and so can be compared to theory.
When C Low-rate is normalised to area, it is equal to the product of the low-rate volumetric capacitance of the device (C V ) times the device thickness (25 nm). For a nanosheet network, it has been shown that 4 where P is the network porosity and <t> is the mean nanosheet thickness ( and  r have the same values as above). Taking <t>=10 nm (see AFM data) and assuming a porosity of 30%, we find C V =12.4 F/cm 3 . Taking a device thickness of 25 nm yields a predicted low-rate areal capacitance of 3.1 F/cm 2 . This is very close to the value of C Low-rate =2.8 F/cm 2 extracted from fitting the data above which shows that the capacitances values in Supplementary Figure   8b are in line with expectations. Then, taking v at 50 mV/s (in line with scan rates used in our transfer and output characteristics) we estimate the C device as ~ 3.1 μF cm -2 comparable to previous reports. [17][18][19] The value of C device need to be scaled for our WS 2 , WSe 2 and MoS 2 devices used in Figure 2 and 5 for the main text respectively to account for the different network thicknesses shown in Supplementary Figure 5. We find C device of ~ 3.1 μF cm -2 and ~ 4.9 μF cm -2 for our WS 2 and WSe 2 devices respectively. We also find C device of ~ 3.1 μF cm -2 , ~ 3.1 μF cm -2 and ~ 1.2 μF cm -2 for our MoS 2 networks of <L> ~ 1040, <L> ~ 605 nm and <L> ~ 484 nm used in our <L> dependence study in Figure 4 (main text).  Table summarising solution processing literature on organic polymers, metal oxides, graphene and CNTs used in figure 3c of the main text.